The invention relates to a method and an apparatus for compensating component-stipulated signal distortions in an optoelectronic apparatus for identifying marks provided with defined contrast patterns, such as bar code symbols. Such an apparatus includes a transmitting element that emits transmitted light, and a receiving element. The transmitted light is guided across the marks and the received light reflected by the marks has an amplitude modulation imposed by the contrast of the marks, wherein the received light is converted in the receiving element into a voltage signal that corresponds to the received signal. The voltage signal is fed to a threshold-value unit for evaluation of the contrast patterns.
An apparatus of this type, which is used to read bar code symbols, is known from JP4-346184A. Such apparatuses can preferably be configured as hand-held reading devices that are guided across the bar code symbols to be read at relatively short distances.
For this purpose, the transmitting element includes, in addition to a transmitter preferably configured as a laser, a scanner that periodically diverts the transmitted light and guides it multiple times across the bar code symbols to be read.
In this type of application, it is sufficient that the transmitted light beam is guided across the bar code at a relatively low scanning rate. Typical scanning rates lie within a range of approximately 30-50 scans/second.
The received light reflected by the bar code symbols has an amplitude modulation that corresponds to the light-dark transitions of the bar code symbols. The signal frequencies are a function of the bar code pattern, on the one hand, and the scanning rate and the reading distance on the other.
The received light is converted into a voltage signal and amplified in the receiving element, which is typically formed by a photosensor, preferably a photodiode, and an amplifier.
The analog received signals are evaluated in the threshold-value unit. The voltage signals are preferably evaluated with a switching threshold, because of which the light zones of the bar code symbols, to which a high intensity of the received signal corresponds, can be distinguished from the dark zones of the bar code symbols, to which a low intensity of the received signal corresponds.
Because the scanning rates at which the bar code symbols are scanned are relatively low, and the reading distances are relatively small and are only subjected to low fluctuations, the bar code symbols can be resolved from the transmitted light beam of the apparatus, i.e., the difference in intensity between the received signals reflected by the light and dark surfaces, respectively, is considerably greater than the signal distortions that take place in the components of the receiving elements. In this respect, reliable detection of the bar code symbols is assured without measures imposed by the manufacturer to compensate component-stipulated signal measurements.
An apparatus for identifying bar code symbols whose receiving element is formed by a line camera comprising a linear arrangement of photodiodes is known from U.S. Pat. No. 4,323,772.
The analog received signal registered by the line camera is digitized by means of a threshold-value unit. A digital filter is disposed downstream of this threshold-value unit. An evaluation of the signals is performed with the digital filter in that a certain state, i.e., a black or white line element of a bar code symbol, is only considered identified when at least two adjacent photodiodes deliver the same received signal. In this way, malfunctions of individual photodiodes can be compensated, or minor errors in the bar code symbols can be suppressed.
However, in many cases, in the use of optoelectronic apparatuses of the type mentioned at the outset in industrial settings, higher requirements are placed on the resolution capability of the apparatus. The distance of the apparatus from the bar code symbols can lie within a range of up to a few meters, and can possibly vary dramatically.
For example, bar code symbols can be applied to packages of different sizes that are transported on a conveyor belt. The optoelectronic apparatus is preferably disposed at a fixed distance above the conveyor belt. Depending on the speed of the conveyor belt, the reading distance and size of the bar code symbols, scanning rates of the apparatus which lie within a range of 1000 scans/second can be necessary. Typical scanning rates lie Within a range of 300-1000 scans/second. At scanning rates of this order of magnitude, the received signal frequencies lie within a range of 0.5 Mhz and higher.
In these types of applications, the signal adulterations caused primarily by the components of the receiving element are of the same order of magnitude as the differences between the received signals reflected by the light and dark surfaces, respectively, of the bar code symbols. A possible result of this is that the bar code symbols can no longer be decoded error-free by the optoelectronic apparatus.